U.S. patent number 7,341,807 [Application Number 11/362,533] was granted by the patent office on 2008-03-11 for non-flammable nonaqueous electrolyte solution and lithium ion cell using same.
This patent grant is currently assigned to Japan Aerospace Exploration Agency. Invention is credited to Kenichi Kuwajima, Saburo Kuwajima, Yoshitsugu Sone, Xianming Wang.
United States Patent |
7,341,807 |
Wang , et al. |
March 11, 2008 |
Non-flammable nonaqueous electrolyte solution and lithium ion cell
using same
Abstract
Disclosed are a non-flammable nonaqueous electrolyte solution
and a lithium ion cell using the electrolyte solution. The
non-flammable nonaqueous electrolyte solution comprises a ternary
or higher-order compound additive, a high concentration of lithium
salt and a phosphoric ester serving as a primary solvent. The
lithium ion cell comprises a positive electrode containing a
lithium transition metal oxide capable of absorbing and releasing
lithium, a negative electrode containing a carbon-based material
capable of absorbing and releasing lithium, and the above
non-flammable nonaqueous electrolyte solution. The ternary or
higher-order compound additive contains at least one compound
selected from each of the three compound groups consisting of: a
compound group (a) of vinylene carbonate compounds; a compound
group (b) of vinyl acetate compounds, alkyl methyl carbonate
compounds and vinyl ethylene carbonate compounds; and a compound
group (c) of 2-pyrrolidinone compounds, cyclic alkyl compounds and
cyclic pentanone compounds. The non-flammable nonaqueous
electrolyte solution of the present invention can provide enhanced
charge/discharge characteristics to a lithium ion cell.
Inventors: |
Wang; Xianming (Tsukuba,
JP), Sone; Yoshitsugu (Tsukuba, JP),
Kuwajima; Saburo (Tsukuba, JP), Kuwajima; Kenichi
(Kanagawa, JP) |
Assignee: |
Japan Aerospace Exploration
Agency (Tokyo, JP)
|
Family
ID: |
34204223 |
Appl.
No.: |
11/362,533 |
Filed: |
February 27, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060240329 A1 |
Oct 26, 2006 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCT/JP03/10792 |
Aug 26, 2003 |
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Current U.S.
Class: |
429/328; 429/331;
429/332; 429/330 |
Current CPC
Class: |
H01M
6/168 (20130101); H01M 10/4235 (20130101); H01M
10/0525 (20130101); H01M 6/166 (20130101); H01M
10/0569 (20130101); H01M 10/0567 (20130101); H01M
10/0568 (20130101); Y02E 60/10 (20130101); Y02T
10/70 (20130101) |
Current International
Class: |
H01M
10/40 (20060101) |
Field of
Search: |
;429/328,329,330,331,332 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2000215911 |
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Aug 2000 |
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JP |
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2000235867 |
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Aug 2000 |
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JP |
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2000348762 |
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Dec 2000 |
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JP |
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2001052743 |
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Feb 2001 |
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JP |
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2001160414 |
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Jun 2001 |
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JP |
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2002203597 |
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Jul 2002 |
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JP |
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2002280061 |
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Sep 2002 |
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JP |
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2003234127 |
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Aug 2003 |
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JP |
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Primary Examiner: Kalafut; Stephen J.
Attorney, Agent or Firm: Jacobson Holman PLLC
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
The present patent application is a continuation of International
application No. PCT/JP2003/010792 filed Aug. 26, 2003 and published
in Japanese, which is incorporated herein by reference in its
entirety.
Claims
What is claimed is:
1. A non-flammable, nonaqueous electrolyte solution having a
lithium salt dissolved in a solvent containing a phosphate ester,
comprising at least one additive compound selected from each of the
three compound groups consisting of: (a) vinylene carbonate
compounds represented by formula (I): ##STR00011## wherein R.sup.1
and R.sup.2 each independently represents a hydrogen atom or a
straight-chain or branched-chain alkyl group having 1 to 4 carbon
atoms, (b) a compound selected from the groups consisting of vinyl
acetate compounds represented by formula (II), alkyl methyl
carbonate compounds represented by formula (III), and vinyl
ethylene carbonate compounds of formula (IV): ##STR00012## wherein
R.sup.3, R.sup.4 and R.sup.5 each independently represents a
hydrogen atom or a straight-chain or branched-chain alkyl group
having 1 to 4 carbon atoms, ##STR00013## wherein R.sup.6 represents
a hydrogen atom or a straight-chain or branched-chain alkyl group
having 1 to 4 carbon atoms, ##STR00014## wherein R.sup.7, R.sup.8,
R.sup.9, R.sup.10, R.sup.11 and R.sup.12 each independently
represents a hydrogen atom or a straight-chain or branched-chain
alkyl group having 1 to 4 carbon atoms; and (c) a compound selected
from the group consisting of 2-pyrrolidinone compounds represented
by formula (V), cyclic alkyl compounds represented by formula (VI),
and cyclic pentanone compounds represented by formula (VII):
##STR00015## wherein R.sup.13 represents a hydrogen atom or a
straight-chain or branched-chain alkyl group having 1 to 4 carbon
atoms, ##STR00016## wherein R.sup.14 represents a straight-chain or
branched-chain alkylene group having 2 to 4 carbon atoms,
##STR00017## wherein R.sup.15 represents a straight-chain or
branched-chain alkylene group having 1 to 3 carbon atoms; wherein
each of compounds (a), (b), and (c) is present in an amount from 1
to 12% by weight and the total amount of compounds (a), (b), and
(c) is from 5 to 20% by weight; and further wherein the total
concentration of the lithium salt in the phosphate ester-based
solvent is in the range of 1.5 to 2.5 mol/dm.sup.3; and further
wherein the solvent comprises a phosphate ester selected from the
group consisting of the chain phosphate esters of formula (IX) and
the cyclic phosphate esters of formula (X), wherein at least either
one of the chain phosphate ester represented by formula (IX) and
the cyclic phosphate ester represented by formula (X) is contained
in the phosphate ester-based solvent in an amount ranging from 50
to 100% by volume: ##STR00018## wherein R.sup.16, R.sup.17 and
R.sup.18 each independently represents an alkyl group having 1 or 2
carbon atoms, in which each of the hydrogen atoms may be
substituted with fluorine, ##STR00019## wherein R.sup.19 represents
an alkyl group having 1 or 2 carbon atoms in which each of hydrogen
may be substituted with fluorine, and R.sup.20 represents an
alkylene group having 2 to 4 carbon atoms.
2. The non-flammable nonaqueous electrolyte solution of claim 1,
wherein the lithium salt is at least one compound selected from the
group consisting of lithium salts of an inorganic acid formed from
a lithium ion and an anion selected from PF.sub.6 and BF.sub.4, and
lithium salts of an organic acid formed from a lithium ion and an
anion of formula (VIII): ##STR00020## wherein m and n each
independently represent an integer selected from 1, 2, 3, or 4.
3. The non-flammable nonaqueous electrolyte solution of claim 1,
wherein the phosphate ester-based electrolyte solution comprises a
chain phosphate ester having formula (IX), and at least one cyclic
phosphate ester having formula (X): ##STR00021## wherein R.sup.16,
R.sup.17 and R.sup.18 each independently represents an alkyl group
having 1 or 2 carbon atoms, in which each of the hydrogen atoms may
be substituted with fluorine, ##STR00022## wherein R.sup.19
represents an alkyl group having 1 or 2 carbon atoms in which each
of hydrogen may be substituted with fluorine, and R.sup.20
represents an alkylene group having 2 to 4 carbon atoms.
4. The non-flammable nonaqueous electrolyte solution of claim 1,
wherein the phosphate ester-based electrolyte solution further
comprises at least one compound selected from the group consisting
of carbonate-based compounds, lactone-based compounds, ether-based
compounds, sulfolane-based compounds and dioxolan-based
compounds.
5. A lithium ion cell comprising: an electrolyte solution
consisting of the non-flammable nonaqueous electrolyte solution of
claim 1; a positive electrode containing a lithium transition metal
oxide capable of absorbing and releasing lithium; and a negative
electrode containing a carbon-based material capable of absorbing
and releasing lithium.
Description
TECHNICAL FIELD
The present invention relates to a non-flammable nonaqueous
electrolyte solution and a lithium ion cell using the electrolyte
solution, and more specifically to a non-flammable nonaqueous
electrolyte containing a ternary or higher-order compound additive
for improving charge/discharge characteristics of a lithium ion
cell, a high concentration of lithium salt and a cyclic or
phosphoric ester compound having solvent at a high composition
ratio, and a lithium ion cell using the electrolyte solution.
BACKGROUND ART
A lithium ion cell having a high energy density and a high
operating voltage has rapidly come into wide use as a power source
for mobile or portable devices, such as portable phones,
notebook-size personal computers and video cameras. Further,
various researches for practical application to a satellite, a
rocket, an electric vehicle and a nighttime-electric-power storage
system are being conducted.
A lithium ion cell employs a carbon-based material and a lithium
transition metal oxide, such as LiCoO.sub.2, respectively, in
negative and positive electrode, and has an operating voltage of 4
V or more. Thus, an electrolyte solution for lithium ion cells is
required to have electrochemical stability even in an operation at
4 V or more. A nonaqueous electrolyte solution prepared by
dissolving an electrolyte, such as lithium fluorophosphate
(LiPF.sub.6), in a mixture of carbonate-based nonaqueous solvents,
such as ethylene carbonate (EC), diethyl carbonate (DEC), ethyl
methyl carbonate (EMC) has been developed to meet such a
requirement, and put into general use.
However, due to relatively low flash or inflammation points of
these nonaqueous solvents, the nonaqueous electrolyte solution
involves a problem about safety, firing or explosion likely to be
caused by a wrong operation, such as short-circuiting, overcharge
or over-discharge. As measures against this problem, it has been
proposed to prepare a nonaqueous electrolyte solution using a
mixture of a fluorinated solvent and an organic phosphate compound
having no flash point. For example, Japanese Patent Laid-Open
Publication Nos. 2000-235867 and 2002-280061 disclose the use of an
organic phosphate compound, such as trimethyl phosphate or triethyl
phosphate. However, if this organic phosphate compound is applied
to a lithium ion cell, it will be reductively decomposed on a
surface of a carbon-based negative electrode to preclude
charge-discharge functions as a cell. While the charge-discharge
functions can be maintained by mixing an inflammable solvent, such
as carbonate-based solvent, with the an organic phosphate compound
solvent, while limiting a ratio (volume ratio) of the organic
phosphate compound in the mixed solvent to 50% or less, a primary
solvent will consist of a low-flash-point solvent, such a
carbonate-based, lactone-based, ether-based, sulfolane-based or
dioxolan-based solvent, resulting in loss of non-flammability as an
electrolyte solution.
Japanese Patent Laid-Open Publication Nos. 2000-348762 and
2000-215911 discloses the use of a fluorinated solvent. This
fluorinated solvent has a problem about no applicability to a 4
V-class lithium ion cell, due to poor oxidation/reduction
resistances and low solubility relative to lithium salts.
Japanese Patent Laid-Open Publication No. 2002-203597 discloses a
technique of adding vinylene carbonate and/or vinyl ethylene
carbonate into a phosphate ester-based electrolyte solution to
suppress reductive decomposition of an organic phosphate compound.
However, it is practically difficult to fully eliminate reductive
decomposition of an organic phosphate compound based on the
addition of the two compounds, and an obtained cell is not
insufficient in terms of practical usefulness, due to poor
charge/discharge characteristics.
As above, from a standpoint of improving both charge/discharge
characteristics and safety in a cell, there is a strong need for
developing a new mixed additive and employing an optimal type and
concentration of lithium ion salt to obtain a non-flammable
nonaqueous electrolyte solution capable of suppressing reductive
decomposition of phosphate ester on a surface of a carbon-based
negative electrode so as to achieve a lithium ion cell having
enhanced charge/discharge characteristics.
DISCLOSURE OF THE INVENTION
In view of the above problems, it is therefore an object of the
present invention to provide a non-flammable phosphate ester-based
electrolyte solution for a lithium ion cell having excellent
charge/discharge characteristics, and a lithium ion cell using the
electrolyte solution.
In order to achieve the above object, through various researches
for bringing about breakthrough, the inventors found that a
non-flammable nonaqueous electrolyte solution containing a
phosphate ester having solvent at a high composition ratio, in
combination with a newly developed ternary or higher-order mixed
additive and a discovered optimal type and concentration of lithium
salt, can drastically improved charge/discharge characteristics of
a lithium ion cell at astoundingly high level to achieve the above
object. Based on this knowledge, the inventors have finally reached
the present invention.
Specifically, according a first aspect of the present invention,
there is provided a non-flammable nonaqueous electrolyte solution
which comprises a ternary or higher-order compound additive, a high
concentration of lithium salt and a phosphoric ester serving as a
primary solvent. According a second aspect of the present
invention, there is provided a lithium ion cell which comprises a
positive electrode containing a lithium transition metal oxide
capable of absorbing and releasing lithium, a negative electrode
containing a carbon-based material capable of absorbing and
releasing lithium, and the above non-flammable nonaqueous
electrolyte solution. In the first and the second aspect of the
present invention, the ternary or higher-order compound additive
contains at least one compound selected from each of the three
compound groups consisting of: a compound group (a) of vinylene
carbonate compounds represented by the following formula (I); a
compound group (b) of vinyl acetate compounds represented by the
following formula (II), alkyl methyl carbonate compounds
represented by the following formula (III) and vinyl ethylene
carbonate compounds represented by the following formula (IV); and
a compound group (c) of 2-pyrrolidinone compounds represented by
the following formula (V), cyclic alkyl compounds represented by
the following formula (VI) and cyclic pentanone compounds
represented by the following formula (VII),
##STR00001##
wherein R.sup.1 and R.sup.2 each independently represents a
hydrogen atom or a straight-chain or branched-chain alkyl group
having 1 to 4 carbon atoms,
##STR00002##
wherein R.sup.3, R.sup.4 and R.sup.5 each independently represents
a hydrogen atom or a straight-chain or branched-chain alkyl group
having 1 to 4 carbon atoms,
##STR00003##
wherein R.sup.6 represents a hydrogen atom or a straight-chain or
branched-chain alkyl group having 1 to 4 carbon atoms,
##STR00004##
wherein R.sup.7, R.sup.8, R.sup.9, R.sup.10, R.sup.11 and R.sup.12
each independently represents a hydrogen atom or a straight-chain
or branched-chain alkyl group having 1 to 4 carbon atoms,
##STR00005##
wherein R.sup.13 represents a hydrogen atom or a straight-chain or
branched-chain alkyl group having 1 to 4 carbon atoms,
##STR00006##
wherein R.sup.14 represents a straight-chain or branched-chain
alkylene group having 2 to 4 carbon atoms,
##STR00007##
wherein R.sup.15 represents a straight-chain or branched-chain
alkylene group having 1 to 3 carbon atoms.
Preferably, each of the ternary or higher-order compound additives
selected from the above compound groups (a) to (c) is added to the
non-flammable phosphate ester-based electrolyte solution having a
lithium salt dissolved therein, in an amount ranging from 1 to 12%
by weight, wherein the total amount of the additives is in the
range of 5 to 20% by weight.
Preferably, the lithium salt is an inorganic salt formed from a
lithium ion and an anion selected from PF6 and BF4, and/or an
organic salt formed from a lithium ion and an anion selected from
those represented by the following general formula (VIII), and a
total concentration of the lithium salt in the phosphate
ester-based solvent is preferably in the range of 1.5 to 2.5
mol/dm.sup.3,
##STR00008##
wherein m and n each independently represents an integer selected
from 1 to 4.
Preferably, the phosphate ester-based electrolyte solution contains
at least one of a chain phosphate ester represented by the
following general formula (IX) and a cyclic phosphate ester
represented by the following general formula (X), and the chain
phosphate ester and/or the cyclic phosphate ester are contained in
the phosphate ester-based solvent in an amount ranging from 50 to
100% by volume,
##STR00009##
wherein R.sup.16, R.sup.17 and R.sup.18 each independently
represents an alkyl group having 1 or 2 carbon atoms in which each
of hydrogen may be substituted with fluorine,
##STR00010##
wherein R.sup.19 represents an alkyl group having 1 or 2 carbon
atoms in which each of hydrogen may be substituted with fluorine,
and R.sup.20 represents an alkylene group having 2 to 4 carbon
atoms.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a sectional view showing the structure of a coin-shaped
lithium ion cell serving as a test sample.
FIG. 2 is a graph showing a charge/discharge curve of a negative
half-cell using a 1 mol/dm.sup.3 LiPF.sub.6/EC+DEC (1:2)
electrolyte solution in Comparative Example 1.
FIG. 3 is a graph showing a charge/discharge curve of a negative
half-cell using an electrolyte solution prepared by dissolving 1
mol/dm.sup.3 LiBETI in a TMP solvent, in Comparative Example 2.
FIG. 4 is a graph showing a charge/discharge curve of a negative
half-cell using a 1 mol/dm.sup.3 LiBETI/TMP electrolyte solution
added with a compound additive of 2% VC and 8% VA, in Comparative
Example 3.
FIG. 5 is a graph showing a charge/discharge curve of a negative
half-cell using a 2 mol/dm.sup.3 LiBETI/TMP electrolyte solution
added with a compound additive of 2% VC and 8% VA, in Inventive
Example 1.
FIG. 6 is a graph showing a charge/discharge curve of a negative
half-cell using a 2 mol/dm.sup.3 LiBETI/TMP electrolyte solution
added with a ternary compound additive of 2% VC, 8% VA and 2% NMP,
in Inventive Example 2.
FIG. 7 is a graph showing a charge/discharge curve of a negative
half-cell using a 2 mol/dm.sup.3 LiBETI/TMP+GBL (7:3) electrolyte
solution added with a ternary compound additive of 2% VC, 8% VA and
2% NMP, in Inventive Example 11.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention will now be specifically described. Firstly,
a non-flammable nonaqueous electrolyte solution of the present
invention will be described in detail based on a preferred
embodiment thereof.
In the present invention, a compound additive to be added to a
non-flammable phosphate ester-based electrolyte solution contains
at least one compound selected from each of the three compound
groups consisting of: a compound group (a) of vinylene carbonate
compounds represented by the aforementioned formula (I); a compound
group (b) of vinyl acetate compounds represented by the
aforementioned formula (II), alkyl methyl carbonate compounds
represented by the aforementioned formula (III) and vinyl ethylene
carbonate compounds represented by the aforementioned formula (IV);
and a compound group (c) of 2-pyrrolidinone compounds represented
by the aforementioned formula (V), cyclic alkyl compounds
represented by the aforementioned formula (VI) and cyclic pentanone
compounds represented by the aforementioned formula (VII).
Preferably, each of the additives selected from the above compound
groups (a) to (c) is added to the non-flammable phosphate
ester-based electrolyte solution having a lithium salt dissolved
therein, in an amount ranging from 1 to 12% by weight, particularly
in an amount ranging from 2 to 10% by weight. Further, the total
amount of the additives selected from the above compound groups (a)
to (c) is in the range of 5 to 20% by weight, particularly in an
amount ranging from 8 to 17% by weight, with respect to the
non-flammable phosphate ester-based electrolyte solution having a
lithium salt dissolved therein. If each of the additives is added
in an amount of less than 1% by weight, a reductive decomposition
reaction of the phosphate ester on a surface of a carbon negative
electrode cannot be effectively suppressed, to cause difficulty in
sufficiently improving charge/discharge characteristics of a cell.
If each of the additives is added in an amount of greater than 12%
by weight, the additive will be reductively decomposed on the
surface of the carbon negative electrode despite its intended
purpose to cause the risk of undesirable deterioration in
charge/discharge characteristics of a lithium ion cell.
The lithium salt for use in the present invention, which is a
solute of the non-flammable phosphate ester-based electrolyte
solution, is one or more selected from the group consisting of a
lithium salt of inorganic acid and a lithium salt of organic
acid.
Specifically, the lithium salt of inorganic acid in the present
invention includes lithium fluorophosphates (LiPF.sub.6), lithium
fluoborate (LiBF.sub.4) and lithium perchlorate (LiClO.sub.4).
Among them, LiPF.sub.6 and LiBF.sub.4 are preferable in view of
superiority in charge/discharge characteristics of a cell.
The lithium salt of organic acid in the present invention includes
lithium imide salts formed from a lithium ion and an anion selected
from those represented by the aforementioned general formula
(VIII). Among them, LiN (SO.sub.2C.sub.2F.sub.5).sub.2 and LiN
(SO.sub.2CF.sub.3) (SO.sub.2C.sub.4F.sub.9) are preferable in view
of superiority in charge/discharge characteristics of a cell.
Preferably, the lithium salt is dissolved in the phosphate
ester-based solvent to have a concentration ranging from 1.5 to 2.5
mol/dm.sup.3, particularly from 1.7 to 2.2 mol/dm.sup.3, in the
phosphate ester-based electrolyte solution. If a concentration of
the lithium salt in the phosphate ester-based electrolyte solution
is less than 1.5 mol/dm.sup.3, a reductive decomposition reaction
of the phosphate ester on the surface of the carbon negative
electrode cannot be effectively suppressed, to cause difficulty in
sufficiently improving charge/discharge characteristics of a cell.
If a concentration of the lithium salt is greater than 2.5
mol/dm.sup.3, the non-flammable phosphate ester-based electrolyte
solution has an excessively low electric conductivity to case
deterioration in charge/discharge characteristics of a lithium ion
cell under a high charge/discharge rate.
The phosphate ester contained in the non-flammable phosphate
ester-based electrolyte solution in the present invention includes
a chain phosphate ester represented by the aforementioned general
formula (IX), and a cyclic phosphate ester represented by the
aforementioned general formula (X).
Among them, as a specific example of the phosphate ester containing
no fluorine, trimethyl phosphate, dimethyl ethyl phosphate and
ethylene methyl phosphate are particularly preferable because of no
flash point. As a specific example of the phosphate ester
containing fluorine, trifluoroethyl methyl ethyl phosphate and
ethylene trifluoro ethyl methyl phosphate are particularly
preferable because of no flash point.
The phosphate ester-based electrolyte solution in the present
invention may contain a single phosphate ester or may contain two
or more phosphate esters.
In order to improve charge/discharge characteristics of a cell, the
non-flammable phosphate ester-based electrolyte solution in the
present invention may contain an inflammable organic solvent which
is commonly used as a nonaqueous electrolyte solution for secondary
cells. This organic solvent is not limited to a specific type. For
example, the organic solvent includes: a carbonate-based compound,
such as ethylene carbonate, diethyl carbonate, methyl ethyl
carbonate, dimethyl carbonate, propylene carbonate or vinylene
carbonate; lactone-based compound, such as .gamma.-butyrolactone;
an ether-based compound, such as 1,3-dioxane or monogrime; a
sulfolane-based compound, such as sulfolane; a dioxolan-based
compound, such as 1,3-dioxolan; a ketone-based compound, such as
4-methyl-2-pentanone; a nitrile-based compound, such as
acetonitrile, propionitrile, butyronitrile, valeronitrile or
benzonitrile; a halogenated hydrocarbon-based compound, such as
1,2-dichloroethane; methyl sulfamate; dimethyl thioformamide; and
dimethyl sulfoxide; and a mixture thereof. Among them,
.gamma.-butyrolactone and ethylene carbonate are preferable in view
of a high flash point, a high dielectric constant and advantageous
properties for charge/discharge characteristics of a lithium ion
cell.
The chain phosphate ester represented by the aforementioned general
formula (IX) and/or the cyclic phosphate ester represented by the
aforementioned general formula (X) may be contained in a mixture of
the phosphate ester-based solvent and the above inflammable solvent
in an amount ranging from 50 to 100% by volume, more preferably
from 60 to 85% by volume. Less than 50% by volume of the phosphate
ester is likely to cause difficulty in obtaining sufficient
non-flammability.
A lithium ion cell of the present invention will be described in
detail below. The lithium ion cell of the present invention
comprises the above non-flammable nonaqueous electrolyte solution,
a positive electrode containing a lithium transition metal oxide
capable of absorbing and releasing lithium, and a negative
electrode containing a carbon-based material capable of absorbing
and releasing lithium. In the present invention, except for the
compound additive, the lithium salt and the phosphate-based
solvent, a component of the lithium ion cell, such as a positive
electrode, a negative electrode and a separator, is not limited to
a specific material, it may be made of any material used in
conventional lithium ion cells without modification.
For example, a positive-electrode active material constituting the
positive electrode includes a lithium transition metal oxide-based
material, such as lithium manganese oxide (LiMn.sup.2O.sup.4),
lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide
(LiNiO.sub.2) or lithium titanium oxide
(Li.sub.4/3Ti.sub.5/3O.sub.4). The positive electrode is not
limited to a specific shape. For example, the positive electrode
may be a sheet-shaped electrode prepared by mixing a conductive
material and adhesive with the lithium transition metal oxide-based
material according to need, and applying the oxide-based material
or the mixture onto a collector, or may be a pellet-shaped
electrode prepared by subjecting the oxide-based material or the
mixture to a press forming process. A positive-electrode collector
may be made of aluminum or alloy thereof. Among them, aluminum is
particularly preferable in view of lightweight and high
electrochemical stability.
For example, a negative-electrode active material constituting the
negative electrode includes graphite, surface-treated graphite,
amorphous carbon and non-graphitizable carbon (hard carbon). Among
them, surface-treated graphite is particularly preferable in view
of capability to improve stability of phosphate ester, and high
energy density. These negative-electrode active materials may be
used in the form of a mixture of two or more thereof. The negative
electrode is not limited to a specific shape, but may be a
sheet-shaped electrode prepared by mixing a conductive material and
adhesive with the active material according to need, and applying
the active material or the mixture onto a collector, or a
pellet-shaped electrode prepared by subjecting the active material
or the mixture to a press forming process. A negative-electrode
collector may be made of metal, such as copper, nickel or porous
nickel, or alloy thereof. Among them, copper and porous nickel is
particularly preferable in view of high formability to a thin film
and high electrochemical stability.
A material of the separator includes a nonwoven, or a porous film
made of polyolefin, such as polyethylene or polypropylene.
The lithium ion cell of the present invention is not limited to a
specific shape, but may be formed in any suitable conventional
shape, such as a flat shape (button-like shape), a cylindrical
shape or a rectangular shape. FIG. 1 shows one example of a
coin-shaped lithium ion cell using the non-flammable nonaqueous
electrolyte solution of the present invention.
As shown in FIG. 1, in this lithium ion cell 1, a disc-shaped
positive electrode 2 is disposed on the upper side of the cell 1,
and a disc-shaped negative electrode 4 is disposed below the
positive electrode 2 while interposing a disc-shaped separator 3
therebetween. Further, a spacer 5 is disposed below the negative
electrode 4. The positive electrode 2, the separator 3, the
negative electrode 4 and the spacer 5 are stacked in this order to
form a cell body, and housed in a hermetically-sealed coin-shaped
case 6. A spring 7 is interposed between the spacer 7 and a bottom
surface of the case 6 to upwardly bias the cell body with a
laminated structure comprised of the positive electrode 2, the
separator 3, the negative electrode 4 and the spacer 5, so as to
allow a top surface of the positive electrode 2 to be kept in
contact with an upper inner surface of the case 6. A ring-shaped
gasket 8 having a vertically-elongated rectangular shape in section
(in FIG. 1) is disposed in the case 6. The case 6 is formed by
joining a cup-shaped upper case 6a and a cup-shaped lower case 6b
together along their peripheral edges while receiving the cell body
composed of the positive electrode 2, the separator 3, the negative
electrode 4 and the spacer 5, in an inner space of the case 6. The
gasket 8 is provided as a means to hermetically seal the joined
portion between the upper and lower cases 6a, 6b and electrically
insulate between the positive and negative electrodes.
Typically, in the coin-shaped lithium ion cell as shown in FIG. 1,
LiCoO.sub.2 and graphite are used, respectively, in the positive
electrode and the negative electrode. In this case, a charge
reaction is expressed as follows: Positive Electrode:
LiCoO.sub.2.fwdarw.Li.sub.1-xCoO.sub.2+xLi.sup.++xe.sup.- [1]
Negative Electrode: 6C+xLi.sup.++xe.sup.-.fwdarw.LixC.sub.6 [2]
Respective discharge reactions in the positive and negative
electrodes are induced in the opposite directions of the [1] and
[2]. Typically, in a performance evaluation of one of the positive
and negative electrodes, either one of the positive and negative
electrodes is substituted with a lithium metal (Li) electrode as a
counter electrode, and a positive half-cell of Li/LiCoO.sub.2 or a
negative half-cell of Li/graphite is used. The object of the
present invention is to suppress reductive decomposition of the
non-flammable solvent TMP (trimethyl phosphate) on a surface of the
graphite negative electrode so as to provide a lithium ion cell
having enhanced charge/discharge characteristics. In the present
invention, a performance evaluation was conducted using a
Li/graphite negative half-cell. Charge/discharge reactions in the
graphite negative electrode are expressed as follows: Discharge:
LixC.sub.6.fwdarw.6C+xLi.sup.++xe.sup.- [3] Charge:
6C+xLi.sup.++xe.sup.-.fwdarw.LixC.sub.6 [4]
In this cell, a porous film was used as the separator. The
separator was impregnated with the electrolyte solution. As
mentioned above, a case 6 for housing a cell body had an upper case
6a and a lower case. After setting cell components in the case, the
case was hermetically sealed using a crimping machine. A gasket was
also used for ensuring fluid-tightness of the cell.
In a negative half-cell, a positive electrode is substituted with a
lithium metal electrode. In a positive half-electrode, a negative
electrode is substituted with a lithium metal electrode.
While the following description will be made in connection with
specific examples, the present invention is not limited to the
specific example, but the scope of the invention should be
determined by the appended claims and their legal equivalents.
Respective performances of an electrolyte solution and a cell were
evaluated by the following method.
1. Measurement of Electric Conductivity of Electrolyte Solution: An
electric conductivity was measured at 20.degree. C. using a
conductivity meter (CM-20J "Electric Conductivity Meter" produced
by DKK-TOA Co., Japan) and a cell (C-50101B "Cell for Electric
Conductivity" produced by DKK-TOA Co.).
2. Evaluation of Non-Flammability of Electrolyte Solution: A
glass-fiber filter sheet sufficiently impregnated with an
electrolyte solution was hung up vertically, and a lower end of the
filter sheet was heated for 10 seconds by a fire source. The
non-flammability was evaluated based on whether a flame of the
filter sheet disappears immediately after the fire source is
removed.
3. Preparation of Lithium Ion Cell: A positive electrode was
prepared as follows. Acetylene black serving as a conductive
material was homogenously mixed with lithium cobalt oxide
(LiCoO.sub.2) serving as a positive-electrode active material. The
obtained mixture was dispersed in N-methyl-2-pyrrolidinone as a
solvent for fluorine resin serving as adhesive, and stirred. A
weight ratio of lithium cobalt oxide:acetylene black: fluorine
resin was 90:5:5. After the mixture was formed as homogenous
slurry, the slurry was applied onto one surface of an aluminum foil
serving as a collector to obtain a positive electrode sheet. This
positive electrode sheet was placed on a heat plate heated at
80.degree. C., and dried for 10 minutes. Then, the dried positive
electrode sheet was stored in a dry atmosphere.
A negative electrode was prepared as follows. Surface-treated
graphite serving as a negative-electrode active material was
dispersed in N-methyl-2-pyrrolidinone as a solvent for fluorine
resin serving as adhesive, and stirred. A weight ratio of carbon
material:fluorine resin was 95:5. After the mixture was formed as
homogenous slurry, the slurry was applied onto one surface of a
copper foil serving as a collector to obtain a negative electrode
sheet. This negative electrode sheet was placed on a heat plate
heated at 80.degree. C., and dried for 10 minutes. Then, the dried
negative electrode sheet was stored in a dry atmosphere.
Each of the obtained positive and negative sheets was formed into a
disc-shaped electrode having a diameter of 12 mm, using a punching
machine. A polyethylene multilayer film was formed into a
disc-shaped separator having a diameter of 18 mm, using a punching
machine. Further, a lithium metal foil was formed into a
disc-shaped electrode having a diameter of 14 mm, using a punching
machine under a dry argon atmosphere. Finally, under a dry argon
atmosphere, the obtained disc-shaped electrodes and the separator
were hermetically housed in a con-shaped case to obtain a
positive-negative coin-shaped lithium ion cell. The coin-shaped
case had a diameter of 20 mm and a height of 3.2 mm. Further, a
positive lithium half-cell or a negative lithium half-cell was
prepared.
4. Evaluation of Cell Charge/Discharge Characteristics: A cell
charging operation was performed under a constant current-constant
voltage mode, and a cell discharging operation was performed under
a constant current mode. A constant-voltage charge period was set
at 3 hours. A current rate in each of the cell charging and
discharging operations was 0.2 C. A cutoff voltage for the lithium
ion cell was set at 4.2V and 2.5V, and a cutoff voltage for the
negative half-cell was set at 10 mV and 1.5V. An ambient
temperature of the cells was 20.degree. C.
With reference to the following Table 1, each of Comparative
Examples and Inventive Examples will be described. As described
above, the problem in a phosphate ester-based electrolyte solution
for a lithium ion cell arises from difficulty in cell
charge/discharge due to a reductive decomposition reaction of a
phosphate ester on a surface of a carbon-based negative electrode.
As shown in Table 1, a plurality of negative half-cells using
respective electrolyte solutions were prepared, and respective
charge/discharge characteristics based on the electrolyte solutions
were compared with each other.
Each brevity code in the Table 1 represents the following
compound:
LiBETI: LiN (SO.sub.2C.sub.2F.sub.5).sub.2 TMP: trimethyl
phosphate
VC: vinylene carbonate VA: vinyl acetate
NMP: N-methyl-2-pentanone CH: cyclohexane
CP: cyclopentanone VEC: vinyl ethylene carbonate
AMC: alkyl methyl carbonate GBL: .gamma.-butyrolactone
LiPF6: fluorinated lithium phosphate
TABLE-US-00001 TABLE 1 Cell Characteristics *5 Initial Initial
charge/ Electrolyte Solution discharge discharge DCM Solute Solvent
Additive capacity efficiency Ratio Non- Conductivity No *1 (*2) *3
(mAh) (%) (%) *4 Flammability (mS/cm) Inventive 1 2 mol/dm.sup.3
TMP 2% VC + 8% VA 1.58 80.9 88.5 YES 5.6 Example LiBETI 2 2
mol/dm.sup.3 TMP 2% VC + 8% VA + 2% 1.68 86.3 98.5 YES 5.4 LiBETI
NMP 0.98 *6 84.1 *6 99.3 *6 3 2 mol/dm.sup.3 TMP 2% VC + 8% VA + 2%
1.72 86.5 99.4 YES 5.3 LiBETI CH 4 2 mol/dm.sup.3 TMP 2% VC + 8% VA
+ 2% 168 86.1 99.1 YES 5.5 LiBETI CP 5 2 mol/dm.sup.3 TMP 2% VC +
8% VEC + 2% 1.70 84.1 99.0 YES 5.7 LiBETI NMP 6 2 mol/dm.sup.3 TMP
2% VC + 8% VEC + 2% 1.73 85.4 99.5 YES 5.4 LiBETI CH 7 2
mol/dm.sup.3 TMP 2% VC + 8% VEC + 2% 1.64 87.2 99.0 YES 5.6 LiBETI
CP 8 2 mol/dm.sup.3 TMP 2% VC + 8% AMC + 2% 1.69 85.2 99.3 YES 5.3
LiBETI NMP 9 2 mol/dm.sup.3 TMP 2% VC + 8% AMC + 2% 1.72 86.4 99.3
YES 5.4 LiBETI CH 10 2 mol/dm.sup.3 TMP 2% VC + 8% AMC + 2% 1.67
86.2 98.8 YES 5.5 LiBETI CP 11 2 mol/dm.sup.3 TMP + GBL 2% VC + 8%
VA + 2% 1.77 88.9 100 YES 6.3 LiBETI (7:3) NMP 1.12 *6 86.7 *6 100
*6 Comparative 1 1 mol/dm.sup.3 EC + DEC -- 1.73 83 99.3 NON 8.5
Example LiBETI (1:2) 1.10 *6 81 *6 99.6 *6 2 1 mol/dm.sup.3 TMP --
0.24 12.0 32 YES 6.0 LiBETI 3 1 mol/dm.sup.3 TMP 2% VC + 8% VA 1.36
58.2 92.4 YES 5.9 LiBETI *1: Mol concentration to volume of solute
and solvent (*2): Volume ratio between two solvents *3: Weight
ratio to total weight of solute and solvent *4: DCM Ratio
(Discharge Capacity Maintenance Ratio): percentage of discharge
capacity in 10th cycle to discharge capacity in 1st cycle *5:
Charge/discharge characteristics of negative half-cell *6:
Charge/discharge characteristics of lithium ion cell
FIG. 2 shows charge/discharge characteristics based on the
electrolyte solution in Comparative Example 1. This electrolyte
solution is a conventional electrolyte solution of 1
mol/dm.sup.3LiPF.sup.6/EC+DEC (1:2) which is used in a
commercially-available lithium ion cell. As shown in FIG. 2, the
negative half-cell using this electrolyte solution exhibits
excellent charge/discharge characteristics. That is, as seen in the
charge curve C, a capacity in a voltage range of 0.4 V or more
which reflects a side reaction at an electrode has a low value, and
it is proven that the side reaction is effectively suppressed by
using the electrolyte solution in Comparative Example 1. This makes
it possible to obtain high discharge capacity and charge/discharge
efficiency, as shown in the discharge curve D. However, the
electrolyte solution in Comparative Example 1 has inflammability
and low flash point which lead to safety problems.
In Comparative Example 2, an electrolyte solution used therein was
prepared by dissolving 1 mol/dm.sup.3 LiBETI in a TMP solvent.
While this electrolyte solution is non-flammable and excellent in
safety, a reductive decomposition reaction of the TMP solvent
occurs on a surface of the graphite negative electrode, as shown in
FIG. 3 to cause difficulty in sufficiently performing a cell
charging operation. Consequently, Comparative Example 2 exhibits a
low discharge capacity and a low charge/discharge efficiency. While
the characteristic curve C in FIG. 3 indicates a charging
operation, only the reductive decomposition reaction of the TMP
solvent on the surface of the graphite negative electrode is
developed without the cell charging operation. Thus, the cell
cannot be charged. As the result of no cell charge, no discharge
characteristic can be obtained.
In Comparative Example 3, an electrolyte solution used therein was
prepared by adding a binary composite additive of VC and VA to a 1
mol/dm.sup.3LiBETI/TMP electrolyte solution. The electrolyte
solution in Comparative Example 3 has non-flammability. As shown in
FIG. 4, this electrolyte solution can improve charge/discharge
characteristics of the negative half-cell as compared with
Comparative Example 2. However, a reductive decomposition reaction
of the TMP solvent is not sufficiently suppressed during a charging
operation, and a discharge capacity is inferior to that of
Comparative Example 1.
In Inventive Example 1, an electrolyte solution used therein was
prepared by changing a concentration of LiBETI from 1 mol/dm.sup.3
in Comparative Example 3 to 2 mol/dm.sup.3. As seen in FIG. 5, an
increase in lithium concentration makes it possible to drastically
improve charge/discharge characteristics of the negative half-cell
while maintaining non-flammability of the electrolyte solution.
Specifically, any significant side reaction is not observed in an
initial stage of a charging operation, and therefore a large
discharge capacity can be obtained. It is believed that loved that
a BETI.sup.- anion itself is decomposed on the surface of the
negative electrode in a high concentration to advantageously form a
protective film thereon.
In Inventive Examples 2 to 4, respective electrolyte solutions were
prepared by further adding a third additive of 2% NMP, 2% CH or 2%
CP to the electrolyte solution in Inventive Example 1. The
electrolyte solution in Inventive Examples 2 to 4 allows the
negative half-cell to have enhanced charge/discharge
characteristics while maintaining non-flammability. FIG. 6 shows
charge/discharge characteristic curves based on the electrolyte
solution in Inventive Example 2. Each of NMP, CH and CP is used as
a solvent for fluorine resin serving as adhesive for an electrode
active material. The solvent added to the electrolyte solution
makes it possible to reduce change in volume of the electrode
during charging/discharging operations so as to provide
advantageous effects to cell charge/discharge characteristics.
In Inventive Examples 5 to 10, respective electrolyte solutions
were prepared by substituting the additive VA in Inventive Examples
2 to 4 to VEC or AMC. According to the rest results on Inventive
Examples 5 to 10, it was verified that, even if VEC or AMC is used
in place of VA, the negative half-cell can exhibit excellent cell
charge/discharge characteristics while maintaining
non-flammability.
In Inventive Example 11, an electrolyte solution was prepared by
mixing 30% GBL to a TMP solvent. As seen in FIG. 7, this
electrolyte solution makes it possible to maintain non-flammability
and allow the negative half-cell to have more enhanced
charge/discharge characteristics than those of the cell using a
solvent consisting of only the TMP solvent. Further, a lithium ion
cell using this electrolyte solution exhibits charge/discharge
characteristics equivalent to those of a lithium ion cell using the
electrolyte solution in Comparative Example 1.
INDUSTRIAL APPLICABILITY
As mentioned above, the nonaqueous phosphate ester-based
electrolyte solution for lithium ion cells of the present invention
has non-flammability and allows a lithium ion cell to exhibit
excellent charge/discharge characteristics. The present invention
has excellent specific effects on safety and reliability of lithium
ion cells.
* * * * *